Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. A user equipment is enabled to communicate wirelessly in a cellular communications network, wireless communications system, or radio communications system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The user equipment may further be referred to as a mobile telephone, cellular telephone, laptop, Personal Digital Assistant (PDA), tablet computer, surf plate, just to mention some further examples. The user equipment in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipment within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In the context of this disclosure, the expression downlink (DL) is used for the transmission path from the base station to the user equipment. The expression uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
By handover is commonly referred to the procedure of transferring a user equipment being served by a base station to instead be served by another base station. A serving base station is the base station the user equipment is connected to for communication of user data and services. Handover measurements are used to determine which other base station to make handover to. Handover measurements in conventional cellular systems are based on user equipment measurements on downlink pilot signals. Quantities measured are typically different means of signal strength (e.g. RSRP, Reference Signal Received Power) and signal quality (e.g. RSRQ, Reference Signal Received Quality). Each user equipment measures on signals sent from neighbouring base stations, and compares with the corresponding downlink pilot signal received from the serving base station. Neighbouring base station here refers to a base station that is neighbouring in relation to a base station, and more specifically that the neighbouring base station is located so that it has overlapping radio coverage with the base station and thereby may be a handover candidate for a user equipment being served by the base station.
However, the conventional handover procedure will not always be suitable, or possible, to use and there are of different reasons desirable to be able to support also other handover procedures, in particular such where handover measurements are to be performed by a neighbouring base station on a reference signal from a user equipment.